Issue 74

I. Kacharava et alii, Fracture and Structural Integrity, 74 (2025) 193-205; DOI: 10.3221/IGF-ESIS.74.13

the maximum value of tensile load P max (250 kN) was obtained for experimental verification of the strength of the MCJ prototype in the most loaded area with minimum cross-section, where composite contacts metal. This value was three times lower than the calculated one considering the minimum cross-sectional area of the sample of 200 mm 2 . The following geometric parameters of the composite elements were realized for metal-composite joint prototype sample:  area of the regular cross-section S reg = 300 mm 2 (10×30 mm);  area of minimum cross-section S min ≈ 197 mm 2 (6.4×30 mm);  overall dimensions of the sample: 188  253 mm. The geometric parameters of MCJ metal parts were also calculated. The designed sample, shown in Fig. 3, was manufactured and prepared for strength tensile testing.

T ENSILE TESTING OF A METAL - COMPOSITE JOINT SAMPLE

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he purpose of testing the metal-composite joint sample was to study the effect of tensile load on the damage of the composite connecting element in the form of a “loop”. Tensile loads were applied as shown in Fig. 3. Ten load cells were installed on the composite tape (Fig. 5). Special attention was paid to the minimum cross-sectional area of the loop. When the MCJ specimen was loaded, the contact area between the composite “loop” and the metal “tooth” experienced different mechanical effects: unidirectional carbon fibers stretched in the regular area and compressed in contact with the metal. Loading of the MCJ samples was carried out on an LT TM-1000 testing machine with rate 1 kN/s.

Figure 5: Composite element of a metal-composite joint with installed load cells.

After installing the MCJ sample on the testing machine, it was pre-stretched to eliminate backlash and begin directly loading the composite part. After each loading stage, the MCJ sample was disassembled for non-destructive testing of the composite element using acoustic microscopy. The first loading stage was designed to evaluate the tensile stiffness of the loop and carbon fiber materials. Data on initial stiffness were obtained when the loop was stretched to 64 kN, which was ~26% of maximum design load P max (250 kN). Fig. 6a show tensile diagrams at 65 and 100 kN, respectively, corresponding to 26 and 40% of tensile strength. The diagrams show the linear dependence of deformation δ on load P after measuring the structural gaps in the metal-composite joint, which, when stretched, amounted to 0.6–0.8 mm. The linear nature of the tension diagrams also indirectly indicates the absence of any damage to the metal-composite joint structure. The high load-bearing capacity of the MCJ sample was experimentally confirmed by the following loading step. It was performed with loads up to 200 and 300 kN, which corresponded to 80% and 120% of the calculated breaking load P max , resulting in the initial destruction of composite element structure as well as the deformation of metal bolts during connection (Fig. 6b). The deviation of the load-displacement diagram ( P - δ ) from linearity, when the load exceeds 260 kN, indicates the initial formation of damage, including plastic deformation in the bolts securing the metal-composite joint as well as destruction of the composite material in the loop.

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